Multilayer drawn glass fibers are of increasing importance for the transmission of light beams over long distances, especially for light-wave communications. To avoid excessive light losses, it is now common practice to form composite drawn fibers having a glass core of one optical index of refraction and a surrounding cladding glass layer of a lower index of refraction. Single-mode fibers may have a core diameter of only a few microns and an outer diameter of core and cladding of from 10 to 100 times greater; whereas multi-mode fibers may have much larger core diameters, e.g., up to 60 microns, and outer cladding diameters up to about 150 microns. The cladding layer is customarily enclosed by one or more layers of a suitable plastic to provide physical protection for the delicate fiber. Even so, problems of low fiber strength, inadequate fiber durability, and short life have remained.
When a tensile force is applied on the linear axis of the fiber, tension in the outer fiber surface increases substantially. Even when precautions are taken to keep dust particles and moisture from the outer surface of the glass fiber structure, as by immediate application of the plastic coating during manufacture, the fiber is usually somewhat abraded and micro-cracks tend to form on the fiber surface. Since an optical communication fiber may be subject to considerable tension, such micro-cracks propagate readily from the perimeter of the glass surface toward the glass core. In due time the entire fiber becomes substantially weakened and may fracture after a relatively short life that is totally inadequate for communication purposes. The presence of water molecules on the glass perimeter will also enhance crack propagation, increasing the chances of early failure.
One known method of increasing the strength of glass optical communication fibers is to provide surface compression at the cladding surface. Such a technique is discussed, for example, in an article in the Journal of the American Ceramic Society, December 1969, pages 661-664, by D. A. Krohn and A. R. Cooper, then of Case Western Reserve University. This article presents theoretical and experimental data to show that, if the cladding glass is selected to have a lower coefficient of thermal expansion than that of the core glass and if proper attention is paid to glass transition temperatures of the core and cladding, there is a good probability that compressive stresses can be developed to improve fiber strength.
U.S. Pat. No. 3,849,181--Green teaches that certain types of non-optical glass structural fibers, for example high-strength polycrystalline refractory oxide fibers for use in fiber-glass structures, may be strengthened by applying an extremely thin coating of a glassy material to the outer surface of each fiber. However, the patentee is not concerned with problems relating to optical fibers and he specifies that, for these structural fibers, the outer coating must be less than 0.1 micron in thickness. As will become apparent from the detailed description of our invention below, this extremely thin coating would not be effective to provide a satisfactory high-strength optical structure for purposes of our invention.
It has also been previously proposed in general terms to strengthen a composite optical fiber by applying a second sheath over the cladding sheath which has a lower coefficient of thermal expansion than the cladding sheath or of the combination of core and cladding sheath. See for example the German Federal Republic Offenlegungsschrift No. 24 19 786, published Nov. 6, 1975. Reference may also be made to a corresponding English version in Australian Specification No. 493,505, published Oct. 21, 1976. While this is also an advance in the art, it does not teach how to create sufficiently high compressive forces in the outer layer to produce the improved high-strength long-life optical fibers of our invention.